Dark rum gets its colour from oak ageing. The intense tropical heat expands and contracts the barrels and the wood becomes porous, like a sponge, forcing the spirit in and out of the oak. This adds all of the flavours present in wood – sweetness, vanillin, tannins etc.... Rum is often aged in bourbon and sherry barrels and this in turn brings additional flavours - a raisiny, syrupy sweetness in the case of sherry ageing, or toasty vanilla and smoke from bourbon barrel maturation.